FIGS. 18(a)-18(e) and 19(a)-19(e) are sectional views and perspective views illustrating process steps in a method of fabricating a laser diode producing visible light (hereinafter referred to as a visible light laser diode). Reference numeral 1 designates an n type GaAs substrate. An n type GaAs buffer layer 3 is disposed on the n type GaAs substrate 1. An n type AlGaInP lower cladding layer 4 is disposed on the buffer layer 3. A GaInP active layer 5a is disposed on the cladding layer 4. A p type AlGaInP first upper cladding layer 6a is disposed on the active layer 5a. A p type GaInP etching stop layer 15 is disposed on the first upper cladding layer 6a. A stripe-shaped ridge structure is disposed on a part of the etching stop layer 15. The ridge structure comprises a p type AlGaInP second upper cladding layer 6b in contact with the etching stop layer 15, a p type GaInP band discontinuity reduction (hereinafter referred to as BDR) layer 7 disposed on the second upper cladding layer 6b, and a p type GaAs cap layer 8. The stripe-shaped ridge extends in a &lt;011&gt; direction. An n type GaAs current blocking layer 10a is disposed on the etching stop layer 15, contacting opposite sides of the ridge structure. A p type GaAs contact layer 11 is disposed on the top of the ridge and on the n type GaAs current blocking layer 10. An n side electrode 12 is disposed on the rear surface of the substrate 1 and a p side electrode 13 is disposed on the contact layer 11. Reference numeral 9a designates a mask for selective etching.
A description is given of the fabrication method of the visible light laser diode.
Initially, as illustrated in FIG. 18(a), there are successively grown on the n type GaAs substrate 1, the n type GaAs buffer layer 3, the n type AlGaInP lower cladding layer 4, the GaInP active layer 5a, the p type AlGaInP first upper cladding layer 6a, the p type GaInP etching stop layer 15, the p type AlGaInP second upper cladding layer 6b, the p type GaInP BDR layer 7, and the p type GaAs cap layer 8. Preferably, these layers are grown by MOCVD (Metal Organic Chemical Vapor Deposition). Thereafter, a mask pattern comprising SiN or SiON is selectively formed on the p type GaAs cap layer 8 by CVD (Chemical Vapor Deposition), and a photoresist is deposited thereon and patterned using a photolithographic technique to form a stripe-shaped selective mask 9a extending in the &lt;011&gt; direction.
In the step of FIG. 18(c), using the selective mask 9a, the p type GaAs cap layer 8, the p type GaInP BDR layer 7, and the p type AlGaInP cladding layer 6b are selectively etched until the etching front reaches the etching stop layer 15, thereby forming a stripe-shaped ridge. In the etching process, a tartaric acid containing etchant, a chlorine containing etchant, and a sulfuric acid containing etchant are applied to the cap layer 8, the BDR layer 7, and the cladding layer 6b, respectively.
In the step of FIG. 18(d), the n type GaAs current blocking layer 10a is grown contacting the opposite sides of the stripe-shaped ridge, followed by removal of the mask 9a. Thereafter, the p type GaAs contact layer 11 is grown over the entire surface. To complete the laser diode, the n side electrode 12 and the p side electrode 13 are formed by vapor deposition (FIG. 18(e)).
A description is given of the operation. When a forward bias voltage is applied across the n side and p side electrodes 12 and 13, current flows between the electrodes and is concentrated in the ridge structure because reactive current is blocked by the p-n-p structure formed by the p type contact layer 11, the n type current blocking layer 10a, and the p type etching stop layer 15. Electrons and holes injected into the active layer 5 in the ridge structure recombine to produce light. The light thus generated is transmitted along the stripe-shaped ridge, reflected and amplified between opposite cleaved facets of the laser (not shown). When the amplification exceeds a threshold, laser oscillation occurs.
In the above-described fabrication process of the semiconductor laser, since there is no etchant that etches GaInP but does not etch AlGaInP, when the p type GaInP BDR layer 7 is etched with a chlorine containing etchant (FIG. 18(c)), the p type AlGaInP second upper cladding layer 6b is also etched. Therefore, the thickness of the remaining portion of the AlGaInP cladding layer 6b after the etching of the GaInP BDR layer 7 varies between laser diodes simultaneously formed in a wafer or between different positions in a single laser diode. Further, in the subsequent etching of the AlGaInP layer 6b with a sulfuric acid containing etchant, since this etchant has a low selectivity of AlGaInP to GaInP, a portion of the p type GaInP etching stop layer 15 is unfavorably etched away or a portion of the AlGaInP cladding layer 6b remains unetched. Consequently, in the prior art production method, it is very difficult to produce the ridge structure with high accuracy.
Further, in the prior art semiconductor laser, current injected into the ridge structure flows in the resonator length direction, i.e., the transverse direction, at the junction of the ridge and the etching stop layer 15. This current spreading in the transverse direction becomes an obstacle to reduction of the threshold current and increase in the output power of the laser diode.